A hologram with which two-dimensional data can be recorded with high density attracts attention for the purpose of high-density information recording. The feature of the hologram is that the wave surface of light which carries information to be recorded is voluminally recorded in the form of a change in refractive index in a hologram recording medium composed of a photosensitive material.
Heretofore, as a digital recording system to which the principles of holography is applied, for example, there is one which records or reproduces light or dark light dot pattern data in or from a recording medium made of a photo refractive crystal such as lithium niobate. In a holographic memory system, data can be recorded or reproduced in pages each having a two-dimensional plane, and also multiple recording can be made by utilizing a plurality of pages of the recording medium. In a recording medium which is a kind of Fourier Transformation hologram, data is dispersively recorded in two-dimensional image pages within a three-dimensional space. Hereinafter, an outline of a recording/reproducing system will be described.
In FIG. 1, an encoder 25 converts digital data to be recorded in a recording medium 1 into a light and dark dot pattern image on a plane, and rearranges the dot pattern image into data arrangement of, for example, 480 bits in length×640 bits in width, thereby generating two-dimensional page data. The data is sent, for example, to a spatial light modulator (SLM) 12 such as a panel of a transmission type liquid crystal display (LCD).
The spatial light modulator 12 has a modulation processing unit of 480 pixels in length×640 pixels in width corresponding to a unit page, and optically modulates light irradiated thereto into a spatial optical ON/OFF signal according to the page data sent from the encoder 25 and guides the signal as signal light to a lens 13.
The signal light is made incident to the recording medium 1 through the lens 13. In addition to the signal light, reference light is made incident at an incident angle β with respect to a predetermined reference line perpendicularly intersecting with a beam optical axis of the signal light to the recording medium 1 .
The signal light and the reference light interfere with each other in the recording medium 1, and resulting interference fringes are stored in the form of a refractive index grating in the recording medium 1, thereby recording the data in the recording medium 1. In addition, the angle multiple recording of a plurality of two-dimensional plane data is performed by making the reference light incident to the recording medium 1 by changing the incident angle β, thereby allowing a large amount of information to be recorded.
When the recorded data is reproduced from the recording medium 1, only the reference light is made incident at the same incident angle β as that at the time of recording the refractive index grating in the recording medium 1. That is to say, unlike at the time of recording, no signal light is made incident to the recording medium 1. As a result, diffracted light from the refractive index grating recorded in the recording medium 1 is guided to an optical detector 22 such as a charge coupled device (CCD) through a lens 21. The optical detector 22 converts the intensity of the incident light into a level of an electrical signal, and outputs the analog electrical signal having a level corresponding to the luminance of the incident light to a decoder 26. The decoder 26 compares the analog signal with a predetermined amplitude value (slice level) and reproduces the data of “1” and “0” corresponding thereto.
With regard to a conventional two-dimensional modulation method for hologram recording, there is one in which when information (page data) is determined which is recorded in the form of signal light by passing non-modulated coherent light beam through the spatial light modulator 12, four or a multiple-of-four number of adjacent pixels of the spatial light modulator 12 are grouped in one set, and a quarter of the pixels constituting each group transmits light, and three-fourth of them shields light (refer to Japanese Unexamined Patent Publication No. Hei 9-197947).
The minimum pattern of the pixels of the spatial light modulator 12 with the two-dimensional modulation method for hologram recording, as shown in FIG. 2(a), consists of four parts. Only one part of them transmits light, and the other three parts of them shield light. There are four case classifications relating to which of the parts transmits light; the minimum pattern corresponds to 2 bits. The 2 bits are expressed depending on which of the parts transmits light. A bit string in FIG. 2 is one string, and other expressions of two bits are also possible. In FIG. 2, an open pixel indicates a pixel which “transmits light,” and a black pixel indicates a pixel which “transmits no light.”
When a bit string consisting of a plurality of “1” and “0” (FIG. 2(b)) is recorded, an arrangement as, for example, shown in FIG. 2(c) is taken on the spatial light modulator 12. Reading process proceeds from the top left side to the right, moves to a left end when reaching a right end and then moves one stage (two pixels) down and moves rightward again. The light which has penetrated through the spatial light modulator 12 is condensed by the lens and forms together with the reference light interference fringes in the recording medium, which are recorded in the recording medium.